The present invention relates to obscurant devices and more particularly to those capable of providing screening against the visual, infrared and millimeter wave regions of the electromagnetic spectrum.
It has long been a desire to increase the survivability of friendly forces in battle by screening them from enemy sensors. Historically, smoke has been used to achieve this aim. However, advances in the field of sensor technology has increased the effectiveness of many weapons systems by equipping them with surveillance and target acquisition (STA) devices which can exploit the infrared and millimeter wavebands of the electromagnetic spectrum. Longer wavelength radiation is readily transmitted through conventional visual obscurant screens thereby exposing friendly forces to greater risks.
Research has shown that there is currently no single material that is capable of screening effectively at visual, infrared and millimeter wavelengths. Since obscurant materials screen radiation whose wavelength is roughly equal to their particle size it is highly improbable that a single material capable of screening across the millimeter to infrared range will be developed in the near future. In order, therefore, to provide protection against STA devices it is necessary to deploy a mixture of obscurants, for example powders, fibres and pyrotechnic compositions, from a single munition.
There is currently no xe2x80x9ccommercial off the shelfxe2x80x9d device which comprises a mixture of components designed to counter STA devices. However, a design for such a device was disclosed in the Smoke/obscurants Symposium, Apr. 28-30, 1998, Aberdeen Proving Ground, Maryland, USA; The Evolution of a Design for a Rapid Bloom Multi -Spectral obscurant Munition by P J D Collins, J M B Christofi, N Davies and D Green. A disadvantage of this design is that it would tend to be relatively large and complex and therefore expensive to manufacture. A further disadvantage of this device is that the munition has a section with a calibre that is larger than the standard US and UK calibre.
The only known millimeter wave screening munition is the United States M81 66 millimeter grenade (NATO Classification; Grenade Launcher Smoke: MM/IR screening M81). A disadvantage of this grenade is that, although the design is capable of carrying some infrared screening payload, it is optimised for performance in the millimeter waveband. In practice, in order to achieve multi-spectral screening the US require the use of a number of different obscurant devices, e.g. one for infrared screening, one for visual screening and one (the M81) for millimeter screening.
It is therefore an object of the present invention to provide an obscurant device which alleviates some of the above disadvantages by constructing part or all of the device payload casing from a material that contributes to the screening effect of the device.
Accordingly, the present invention provides an obscurant device comprising an obscurant payload, a detonator, a burster charge which is initiated by action of the detonator and which is capable of disseminating said payload and a payload casing wherein some or all of the payload casing is configured to disintegrate upon actuation of the burster charge and to act thereafter as an obscurant.
Usefully the payload casing can be configured to provide effective electromagnetic screening in the millimeter waveband by constructing the casing out of a conductive carbon fibre. In this context effective millimeter wave attenuation is taken to be xe2x89xa710 dB (xe2x89xa610% transmission) for a single pass through an obscurant cloud.
Suitable fibre types for construction of the casing include:
i) UTS carbon fibre, a PAN (poly-acrylo-nitrile) based carbon fibre which has a Young""s Modulus (YM) of 230 Gpa;
ii) Nickel coated carbon (Nixe2x80x94C), a PAN based carbon fibre with a YM similar to UTS;
iii) UD cloth carbon (UD-C), a unidirectional non-crimp material using carbon with a YM=230 Gpa;
iv) J-UTS carbon fibre, similar to the UTS fibre above but with a higher strain to failure;
v) P100s carbon fibre, a pitch based carbon fibre with higher electrical conductivity than that observed for PAN-based fibres;
vi) Ultra-high Modulus (UMS) carbon fibre, a high modulus PAN-based carbon fibre.
It was found in tests that highest mean attenuation in the measured millimeter wavelengths was achieved when the casing was made from VMS carbon fibre.
In order to achieve attenuation at the required frequencies the conductive casing should disintegrate into fibre lengths in the range of 1 mm to 10 mm. This is because the level of attenuation is maximised when the fibre-length is approximately a half-wavelength. For example, at 94 GHz (=3 mm) a fibre length of 1.5 mm is required.
Furthermore, manufacture of the payload casing can conveniently be achieved by dry filament winding as described more fully hereinafter. The Applicant has found that manufacture of the payload casing by the above technique using commercially available carbon fibre naturally results in a structure that disintegrates upon detonation into individual fibres suitable for millimeter screening. Suitable carbon fibre can be obtained from, for example, the following companies Tenax Plastics Limited, Akzo, Amoco, Courtaulds and Roskill.
Conveniently, the device can carry a mixture of obscurants as payload in order to result in screening at multiple wavebands. For example, if the device carries a brass flake/red phosphorous payload then, in addition to the millimeter screening effect generated by the disintegrating payload casing, the device also screens in the infrared and visual wavebands.
A device as described above can conveniently be adapted for use as a munition or as a decoy flare for deployment from an aircraft or a ship. At present aircraft and ships use different infra-red and radar decoys. For use in aircraft the device described above would be loaded with a magnesium/teflon/viton (MTV) payload and for naval uses a payload of red phosphorous would be appropriate.